Page:The System of Nature.djvu/144

 from which the brain derives distinct perceptions, which breed in it a diversity of ideas; it is this that constitutes the touch or feeling.

The delicacy of the membrane by which the interior of the nostrils is covered, renders them easily susceptible of irritation, even by the invisible and impalpable corpuscles that emanate from odorous bodies: by these means sensations are excited, the brain has perceptions, and generates ideas: it is this that forms the sense of smelling.

The mouth, filled with nervous, sensible, movable, irritable glands, saturated with juices suitable to the dissolution of saline substances, is affected in a very lively manner by the aliments which pass through it for the nourishment of the body; these glands transmit to the brain the impressions received: perceptions are of consequence; ideas follow: it is from this mechanism that results taste.

The ear, whose conformation fits it to receive the various impulses of air, diversely modified, communicates to the brain the shocks or sensations; these breed the perception of sound, and generate the idea of sonorous bodies: it is this that constitutes hearing.

Such are the only means by which man receives sensations, perceptions, and ideas. These successive modifications of his brain are effects produced by objects that give impulse to his senses; they become themselves causes, producing in his soul new modifications, which are denominated thought, reflection, memory, imagination, judgment, will, action; the basis, however, of all these is sensation.

To form a precise notion of thought, it will be requisite to examine, step by step, what passes in